For maths, examsolutions.net is a great site. I use it for A Level maths but pretty sure it does GCSE. The worked through exam questions (video solutions) are specific to your examboard and are organised by topic which is really useful. For science, I'm not too sure but I use PhysicsAndMathsTutor (website) for A Level physics past paper questions which are, again, very well organised etc. I think they also do GCSE (and maybe chemistry and bio too... but would have to double check).
For maths, examsolutions.net is a great site. I use it for A Level maths but pretty sure it does GCSE. The worked through exam questions (video solutions) are specific to your examboard and are organised by topic which is really useful. For science, I'm not too sure but I use PhysicsAndMathsTutor (website) for A Level physics past paper questions which are, again, very well organised etc. I think they also do GCSE (and maybe chemistry and bio too... but would have to double check).